High Voltage Direct Current (HVDC) technology can be expected to play key part in future electrical grids. As the HVDC technology is maturing and the number of installed DC links increases, the need and demand for improved DC switchgears is likely to arise. A gas-insulated switchgear (GIS) has much reduced physical dimensions compared to more traditional open-air switchgear facilities and is likely to attract interest as a potential candidate for such improved DC switchgears, in particular as the GIS also has increased safety compared to the traditional switchgear facilities.
In order to connect a power cable to a GIS system a GIS cable termination is needed. For alternating current (AC) such terminations are available in a wide range of voltage levels. In contrast, for DC no such high voltage GIS cable terminations are available. The constrained dimensions of a GIS system make the realization of a robust GIS cable termination difficult, in particular in view of electrical phenomena unique for DC. For example, in comparison with cable terminations for AC applications, cable terminations for DC applications encounter more difficult electric fields and these cable terminations thus has to be designed in view of being able to cope with DC fields. In addition, there are general difficulties in increasing the transmission power for DC cable systems, e.g. requiring the DC solutions to be able to handle thermal issues resulting from higher currents leading to higher temperatures. The higher voltages also entail exposing the cable terminations to higher electric fields, which dramatically increases the risk of material breakdown.
Insulation and mechanical performances are also important when designing cable terminations and the electric field has to be controlled carefully on various critical locations.
Currently available DC cable terminations are free-standing in air and either comprises solid/rubber materials at lower voltages, or fluid-filled/porcelain insulators at increased voltages. Free-standing DC terminations require a large footprint due to long flashover distances in air.